Polysaccharide derivatives are typically produced as a reactor product in a form which is friable or lumpy or may resemble cotton wool. The reactor product is typically purified by washing. In this form, the wet polysaccharide derivatives still retain residual structures determined by the raw materials. Thus, for example, cellulose ethers can still exhibit the fibrous structure of the initial cellulose. These polysaccharide derivatives are generally unsuitable for use, for example, as products soluble in organic and/or aqueous media. In principle, virtually all polysaccharide derivatives have therefore to be ground and dried in order to be made suitable for use.
The cellulose derivatives rank among the industrially important polysaccharide derivatives. Their preparation, properties and applications are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, (1986), Volume A5, pages 461-488, VCH Verlagsgesellschaft, Weinheim or in “Methoden der organischen Chemie” (methods of organic chemistry), 4th Edition (1987), Volume E20, Makromolekulare Stoffe, Part Volume 3, pages 2048-2076, Georg Thieme Verlag, Stuttgart.
EP-B 0 370 447 (equivalent to U.S. Pat. No. 4,979,681) describes a process for the non-destructive grinding and simultaneous drying of moist cellulose ethers, wherein a cellulose ether having an initial moisture content of 20 to 70 wt. % is conveyed by means of a transport gas and simultaneously comminuted by impact and friction and, by means of the energy of grinding, is dried to a residual moisture content of 1 to 10 wt. %.
WO 96/00748 discloses a process for comminuting cellulose ethers which involves the extrusion of a hydrated cellulose ether through orifices having a cross-sectional area of from 0.0075 mm2 to 1 mm2 (7.5×10−9 m2 to 1×10−6 m2) and cutting up the extrudate thus produced to the desired length.
These prior art processes are mostly multi-step with a predryer or preliminary embrittlement or preliminary compaction. Furthermore, in all processes the chemical and/or thermal action on the macromolecules, particularly during the processing of highly viscous, highly substituted products, is invariably so high that during the grinding process the macromolecules are broken down in the sense that their chains are reduced in length, which is manifested in particular by a more or less extensive breakdown of the viscosity compared with that of the products initially used. Moreover, the surfaces of the products treated by means of preliminary embrittlement or preliminary drying steps become keratinised.
EP-A 0 954 536 (equivalent to U.S. Pat. No. 6,320,043) discloses a process wherein a) a polysaccharide derivative is soaked or dissolved in an adequate quantity of a solvent like water, preferably 35 to 99 wt. %, particularly preferably 60 to 80 wt. %, based on the total weight, so that primary structures, such as fiber structures, which result from the polysaccharide starting material are largely removed, and subsequently b) the polysaccharide starting material is converted into the solid state in a dryer-pulveriser wherein the solvent contained in the soaked or dissolved polysaccharide derivative is converted into the vapour phase by means of a superheated vapour, and then c) optionally, in a subsequent drying step, is dried to the required moisture content in prior art units. The polysaccharide derivatives prepared by this process have a high bulk density and good flow properties. The particles thus produced have a shape factor of less than 5 and greater than/equal to 1, the majority (>50 wt. %) having a shape factor of less than/equal to 2 and the proportion of fine dust in the product being low. The shape factor means the ratio of the largest diameter to the smallest diameter of an (ideally ellipsoidal) body.
EP-A 1 127 895 (equivalent to U.S. Pat. No. 6,509,461) discloses a process for producing particulate water-soluble cellulose derivatives by a) forming a feed composition comprising 20 to 50 wt.% of a cellulose derivative and 50 to 80 wt.% of water, wherein the cellulose derivative is swollen or dissolved in the feed composition and b) contacting, in a high rotational speed impact mill, the feed composition with a heat exchange gas and carrier gas. EP-A 1 127 895 teaches that the bulk density of the particulate cellulose derivate product can be adjusted by the amount of the water present in the feed composition.
However, the water content of a moist cellulose derivative that is obtained after production and washing of the cellulose derivative is typically not constant. In a continuous process the water content is typically difficult to measure on-line, particularly if the water content is over 60 percent. Accordingly, adjusting the bulk density of a particulate cellulose derivate by measuring the water content in the feed composition for a dry-grinding device is inconvenient. Measurements of the water content by microwave or Near Infrared (NIR) are known for granular moist materials. However, these procedures are mainly useful for granular materials, but moist polysaccharide derivates of a high moisture content often have the shape of a paste or large lumps. Moreover, NIR measurements require extensive calibration.
One object of the present invention is to be able to control one or more critical dimensions of the polysaccharide derivative particles after dry-grinding. The optimum particle dimensions depend on the end-uses of the polysaccharide derivatives. Furthermore, a preferred object of the present invention is to be able to influence the bulk density of the polysaccharide particles after dry-grinding.